It is well known that in electronic image reproduction processing, an image signal obtained by scanning an original synchronized with a sampling clock pulse signal of a specific frequency is, after undergoing an A/D conversion, written into a memory device and then read out therefrom, to be submitted to a subsequent recording process.
There are three principal methods of providing image magnification ratio conversion in the main scanning direction. One method is disclosed in U.S. patent application Ser. No. 924,928, filed on 7-17-78, now abandoned in which the speed of revolution of a recording drum is maintained constant and that of an input drum is made variable according to a predetermined magnification ratio. An original image signal picked up each scanning line is synchronized to a memory write clock pulse signal which is of the same frequency as that of a memory read clock pulse signal. The read and write pulse signals are applied to a memory corresponding to an output of an encoder (or equivalent) that is coaxially connected to an output drum. The memory is read out in synchronism with a read clock pulse signal having a predetermined frequency and phase relative to the reading clock pulse signal, for recording a reproduction image of the predetermined magnification ratio and at a predetermined position.
In another method disclosed in U.S. Pat. No. 3,272,918, the speeds of revolution of an input drum and a recording drum are constant (usually both are the same) independent of the magnification ratio. The frequency ratio of a writing clock pulse signal and a reading clock pulse signal for a memory is varied according to the predetermined magnification ratio with the former being normally at a higher frequency than the latter.
In the third method disclosed in U.S. Pat. No. 4,163,605, U.S. patent application Ser. No. 933,714, and JP Patent Laid Open No. 54-65601, with the speeds of revolution of an input drum and a recording drum being maintained constant independent of magnification ratio, the frequency ratio between the memory write and read clock pulse signals is at unity. The main scanning addresses of the memory are omitted or overlapped in part when image data stored in the memory are read out to be used for recording an enlarged or reduced reproduction image.
In each of the above-mentioned methods, there is observed the appearance of moire, which is a kind of interference between the sampling and reading frequencies of a memory when the former is substantially lower than the latter.
The word "substantially" is used as follows: Even when the writing frequency is equal to the reading frequency in the first example above, the pitch between adjacent sampling areas of the original image is increased in conformity to the increase of speed of revolution of the input drum.
Assuming that in the third example the image data of every second pixel obtained at the input side are applied to the recording side, because the same effect can be brought forth by causing the writing frequency to be half of the reading frequency, the writing frequency is regarded as being reduced by one-half.
Under such circumstances, a process has recently been introduced for reducing moire by varying the diameter of an input scanning beam employed.
However, in order to obtain image data accurately in a lower magnification ratio of image reproduction, a broader aperture must be used in association with increasing the diameter of the beam. However, such an input beam of increased diameter is not compatible with a narrower aperture because of the lack of intensity thereof in the aperture. Therefore, it is also necessary to vary the size of the aperture as well as the diameter of the beam in accordance with changes in the image reproduction magnification ratio. Because such systems are complex, it is difficult to economically produce an aperture and a beam capable of automatically following variations in image reproduction magnification ratio.